Therapy of virus-infected plants by heat treatment

Tomato aspermy virus (TAV) was found infecting more than 90% of chrysanthemum plants collected from several gardens in the Hobart district. The virus is readily transferred by sap inoculation to tobacco (Nicotiana tabacum var. Hickory Prior) in which it causes a severe systemic disease, and real relative concentrations of infective particles in tobacco plants can be measured reliably on Chenopodium amaranticolor, a local lesion host, if certain procedures are adhered to. Tobacco plants may be freed from infection by growing them at a constant temperature of 36 C for between 5 and 30 days. The percentage of cures effected is generally greater if cuttings are taken from plants immediately after treatment and rooted in a mist propagation unit. A decreased rate of viral synthesis is one reason for the effectiveness of the method. Under normal conditions of heat treatment, virus multiplication could never be detected although some indirect evidence, from results with cytokinin applications to treated plants, suggested that limited multiplication still occurred. However, measurable synthesis did take place if heated plants were treated with actinomycin D. These observations, together with some additional information on rates of TAV accumulation in plant tissue under various conditions, support a concept of competition between the virus and its host for substrates and access to ribosomes. As ribosomal concentration falls markedly in plants undergoing heat treatment concomitant with an increase in host protein synthesis, an hypothesis is advanced that viral RNA competes poorly with host messenger RNA for the limited numbers of available ribosomes on which to synthesise their respective proteins. This concept may also be used to account for the successes in obtaining virus-free material from infected plants by meristem culture. The rate of virus inactivation in whole plants at 36°c is much slower than the rate in expressed sap, and faster than the rate in purified virus preparations. Also, the kinetics of inactivation both 'in vivo' and in expressed sap do not fit a reaction of the first order, but one of a higher order, showing that inactivation at high temperature is not solely a direct thermal effect. Large increases in the levels of two enzymes, polyphenoloxidase and ribonuclease, occur during heat treatment and these may directly inactivate TAV. The extent of increase of these heat-induced enzymes may vary greatly with the species and variety of tobacco, thus providing an explanation for many past observations and suggestions implicating a role of the host in heat treatment therapy. Several observations indicates that the cytokinin concentration falls in heated plants. This results in an almost complete cessation of mitosis as well as a decrease in the ionic strength of the cell sap concomitant with a large increase in cell volume. The pH of the cell sap also falls markedly during heat treatment. probably reduce the stability of TAV. These changes The observations on alterations in cell size and mitotic activity are taken to negate the commonly advanced suggestion that heat treatment may be successful because the plant \grows away\" from the virus. The results and some conclusions drawn from them offer several different mechanisms which may be important in achieving virus therapy by heat treatment. They allow one to predict the types of virus infections which might be cured by heat treatment and suggest that exogenous application of certain chemicals to plants may be useful in increasing both plant survival and the chances of virus cure in \"difficult plants.\" A more rational approach to virus therapy by heat treatment should therefore be possible."